1. Field of the Invention
The present invention relates to a technique for adjusting the color of a specific region in a color image optimally. Particularly, the invention relates to a technique for deciding a reproduction color after adjustment in a natural image containing a memory color such as flesh color, sky blue, or grass green, which are remembered as an impression of the image by people, on the basis of the memory color, which is a subject of the adjustment, and reproducing the reproduction color on a region of the memory color in the natural image.
2. Description of the Related Art
When a natural image having a region of a memory color remembered by people needs to be adjusted automatically, there is generally taken a method in which a target color for the memory color, which is a subject of the adjustment, is provided so that the region of the memory color in the natural image is adjusted toward the target color. For example, in a memory color adjusting method described in JP-A-2001-169135 (corresponding to EP1089552), a histogram of pixels having pixel values, which are in a predetermined range of hue, is generated on the basis of an input image. An average value in the memory color region in the image is calculated on the basis of the histogram. Look-up tables, which are generated for every color component so that the difference between the average value and an optimum value set, which is prepared in advance, can be eliminated, are used to perform color adjustment.
On the other hand, Japanese Patent No.3264273 (corresponding to EP1087614) has disclosed a technique in which after a representative color of a memory color region, which is a subject of adjustment, is extracted from an image, a correction value prepared in advance in accordance with each hue is used to perform the adjustment. In this case, a color, which has been already decided on the basis of the correction value stored in a memory in advance, is used as the target color.
The correction value of the region, which is the subject of the color adjustment, in the image varies widely according to the degree of brightness or saturation in the subject region. For this reason, if the related-art method is used, it is difficult to keep balance between the correction value at strong correction and the correction value at weak correction. Even in the case where the correction value is set according to each color region in advance, labor is required for setting the correction value.
In the related-art technique, there is the possibility that the adjusted image may be unnatural compared with the original image before the adjustment though the color can come near to an ideal memory color. In the case where the color is adjusted so as to be widely different from a memory color in the original image, the adjusted image becomes unnatural compared with the original image. When, for example, the color of a sky near a cloudy sky is reproduced as the color of a clear sky, it cannot be said that the reproduction is preferable so long as a special request is not given. The same rule can also apply to a case of flesh color or grass green. When the adjusted image is widely different from the original image, it can be hardly said that the reproduction is preferable. However, if the quantity of color adjustment is weakened in consideration of such circumstances, there is a problem that correction becomes weak, for example, in a case of a general blue sky so that preferable reproduction cannot be carried out well.